EP4198610B1 - Head-up display image generation unit with folding mirror - Google Patents

Description

The present invention relates to a head-up display having an image generation unit with a folding mirror.

A head-up display, also known as a HUD, is a display system that allows the viewer to maintain their line of sight by projecting the content into their field of vision. While such systems were originally used primarily in aviation due to their complexity and cost, they are now also being installed in large-scale production in the automotive sector.

Head-up displays generally consist of an imaging unit or PGU (Picture Generating Unit), an optical unit, and a mirror unit. The imaging unit generates the image using at least one display element. Most modern head-up displays use LCD-based displays (LCD: Liquid Crystal Display) to generate the image. The optical unit directs the image onto the mirror unit. The mirror unit is a partially reflective, translucent pane. The viewer therefore sees the content displayed by the imaging unit as a virtual image and, at the same time, the real world behind the pane. In the automotive sector, the windshield is often used as the mirror unit, and its curved shape must be taken into account in the display. The interaction of the optical unit and the mirror unit results in the virtual image being an enlarged representation of the image generated by the imaging unit.

From the DE 41 02 678 A1 A head-up display for a means of transport is known, comprising an imaging unit for generating an image, an optical unit for projecting the image by a mirror unit onto a virtual image plane, and a folding mirror, the angle of incidence and angle of reflection of which are unequal. This property is achieved, for example, using holographic components, diffraction gratings, or Fresnel mirrors.

Also the US 5,313,326 A , the WO 2014/041689 A , the US 2015/0362221 A1 and the US 2018/252917 A1 show head-up displays that use a folding mirror whose angle of incidence and angle of reflection are unequal.

In these well-known head-up displays, the folding mirror, whose angle of incidence and angle of reflection are unequal, is positioned in the image path. This property can have an adverse effect on image quality and/or sensitivity to stray light.

From the US 2010/0195022 A1 A backlight for a liquid crystal display is known in which one edge of a light guide is provided with a multitude of reflective surfaces that expand a tightly focused laser beam into a wide beam of light. However, the use of a laser light source in head-up displays is mainly known for micro-mirror units (so-called DMDs, or Digital Micro Mirror Devices), which require a light beam with a very small cross-section and a narrow aperture angle.

Also from the US 2015/036215 A1 is a backlight for a liquid crystal display.

From the US 2019/0212560 A1 A head-up display is known in which an optical element comprises a mirror with microstructures that project light onto a display element. A disadvantage is that the illumination of the display element is uneven, with dark stripes.

From the US 2020/0033600 A1 A head-up display is known in which a liquid crystal display element is designed to use as much of the unpolarized light generated by a light source as possible by means of a polarization converter.

From the US 2019/265472 A1 A head-up display for a means of transport is known, comprising: an imaging unit for generating an image; an optical unit for projecting the image by means of a mirror unit; wherein the imaging unit has a folding mirror, the folding mirror is arranged between a light source and a display element illuminated by the light source at an angle of incidence to the propagation direction of the light incident on it from the light source, the folding mirror has microstructures, wherein the microstructures have first mirror surfaces which are arranged at a first angle different from the angle of incidence of the folding mirror and are spaced from one another to form gaps, and wherein second surfaces are arranged at a second angle in the gaps.

A head-up display that is improved compared to the existing head-up displays is desired.

A head-up display according to the invention is specified in claim 1. The head-up display has an imaging unit for generating an image and an optical unit for projecting the image through a mirror unit. The imaging unit has a folding mirror. The folding mirror is arranged between a light source and a display element illuminated by the light source at an angle of incidence to the propagation direction of the light incident on it from the light source. The angle of incidence is preferably less than 45°, which allows for a flatter installation space. The folding mirror has microstructures. The microstructures have first mirror surfaces arranged at a first angle different from the angle of incidence of the folding mirror and spaced from one another to form gaps. Second surfaces are arranged in the gaps at a second angle. A polarizer directs light of a first polarization to the display element and light of a second polarization into the gaps. A retarder converts the polarization of the light directed into the gaps into the first polarization. The light guided into the gaps is then guided towards the display element after passing through the gaps. This has the advantage of achieving polarization recycling and allowing light from the gaps to reach the display element. Normally, only one of the polarizations generated by the light source is Polarization directions are used when the display element is a linearly polarized light modulating display element. This is the case, for example, with liquid crystal displays (LCDs). According to the invention, the normally unused polarization is converted into the polarization required by the display element by means of a polarizer and retarder, exploiting the gaps in the folding mirror and is then passed on to the element. The light directed into and passing through the gaps has the same polarization as the light polarized by the mirror surfaces. The display element is thus illuminated without gaps with light of a single polarization. Ideally, this also eliminates the need for additional homogenization measures. Such an ideal case exists, for example, for certain duty cycles. In a real system, the stripe structure is blurred with scatterers, for example. This type of scattering then also serves to illuminate the eyebox.

According to the invention, the first mirror surfaces have different first angles among one another. Groups of first mirror surfaces with the same first angles can also have different first angles from group to group. In particular, however, each first playing surface has a different first angle than the other first mirror surfaces. This has the effect of the folding mirror being a curved folding mirror, also called a cylindrical mirror. Macroscopically speaking, the folding mirror is not curved; rather, the first mirror surfaces and the second surfaces of the microstructures are arranged such that the folding mirror acts like a cylindrical mirror. In this way, a fanned-out radiation or astigmatism is achieved, which is desirable in certain circumstances of a head-up display.

According to the invention, the polarizer is designed as a reflective polarizer and arranged between the folding mirror and the display element. The retarder, according to this variant, is a retarder that converts linear polarization into circular polarization and is passed through by the light twice. Such a retarder is also called a λ/4 plate or quarter-wave plate. Here, the effect of a lambda/2 retarder is split, and the delay is distributed over two passes.

The retarder in this variant is located between the folding mirror and the polarizer. This has the advantage of allowing the components to be designed flat, allowing the use of cost-effective, mass-produced components.

The input-side polarizer of a display element that requires polarized input light, such as a liquid crystal display, is advantageously used as the polarizer. This saves components and, due to the fewer interfaces the light must pass through, also results in lower light output losses and less interference.

According to this variant, the polarizer and retarder are also advantageously designed as a single piece as a reflective circular polarizer.

Advantageously, the reflective polarizer is inclined at an angle other than 90° to the propagation direction of the light incident on it from the folding mirror, and the reflective second surfaces are arranged parallel to the reflective polarizer. This has the advantage that light reflected by the reflective polarizer, which is not or hardly divergent, is not reflected back onto the first mirror surfaces, but rather, with a suitable choice of angle and spacing, onto one of the second reflective surfaces in the gaps. From these, it is reflected parallel to the light reflected by the first mirror surfaces towards the polarizer, from which it is transmitted after the polarization direction has been adjusted by means of the retarder. Thus, a very large proportion of the light of both polarizations is utilized, and almost no dark areas are caused by the gaps. The display element is illuminated very homogeneously.

According to a variant of the invention, the folding mirror is part of a transparent body with a wedge-shaped cross-section, in which the wedge base is the light entry surface facing the light source, the microstructures are arranged on one of the large side surfaces of the wedge, and the other large side surface of the wedge is the light exit surface facing the display element. This has the advantage that the transparent body is a large-volume component. This is less susceptible to damage than very thin components, simplifying handling during production. The transparent body, for example, is cast in a large mold and then provided with polarizer and retarder functionality on the light-emitting surface, for example, with an appropriate coating. These are proven manufacturing processes that deliver reliable results.

According to the invention, reflective second surfaces are provided, each paired to form a retroreflector. Such an arrangement is robust against angular tilt. Even if the folding mirror is not precisely aligned with the incident light, it reflects in such a way that the outgoing light is parallel to the incoming light.

According to the invention, the first mirror surfaces have different angles to one another. This can be done in groups, as stated above. According to a preferred variant, each of the first mirror surfaces has a different first angle, and the reflective polarizer has first polarizer surfaces and second polarizer surfaces, each of which forms a retroreflector in pairs. Thus, the light fanned out by the first mirror surfaces is reflected by 180° when reflected by the reflective polarizer, regardless of the angle at which it strikes the reflective polarizer. Reflection by the second surfaces also occurs by 180° if these are also designed as retroreflectors. The angular fanning caused by the first mirror surfaces is thus retained even after this double reflection; the light with rotated polarization then passes through the reflective polarizer at the same angle as the light that was already polarized enough to pass through the reflective polarizer when it first struck it. Here, too, a fanned-out radiation or astigmatism is realized, which is desirable in certain conditions of a head-up display.

Advantageously, the folding mirror and the display element are aligned parallel to each other. Light rays coming from the folding mirror strike the display element therefore, travel a distance of equal length. Uniform illumination of the display element is thus achieved, since deviations caused by differences in distance do not occur in this configuration.

This advantage also occurs when, in a head-up display, a folding mirror with the property that the angle of incidence and the angle of reflection are macroscopically unequal is aligned parallel to a display element of this head-up display.

Further features of the present invention will become apparent from the following description and the appended claims taken in conjunction with the figures.

Figure overview

Fig. 1

shows schematically a head-up display according to the state of the art for a motor vehicle;

Fig. 2

shows schematically the imaging unit of a head-up display;

Fig. 3

shows schematically the imaging unit of a head-up display according to the invention;

Fig. 4

shows schematically a folding mirror of a head-up display;

Fig. 5

shows schematically a part of a first embodiment

Fig. 6

shows schematically a part of a second embodiment

Fig. 7

shows schematically a part of a third embodiment

Fig.8

shows a variant of the first embodiment

Fig.9

shows another embodiment

Fig.10

shows an embodiment with a curved folding mirror

Fig.11

shows an embodiment of the invention with a curved folding mirror.

Character description

To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. Like reference numerals are used in the figures for like or equivalent elements and are not necessarily described again for each figure. It is understood that the invention is not limited to the illustrated embodiments and that the described features can also be combined or modified without departing from the scope of the invention as defined in the appended claims.

Fig. 1 shows a schematic diagram of a head-up display for a motor vehicle according to the prior art. The head-up display has an imaging unit 1, an optical unit 2, and a mirror unit 3. A beam SB1 emanates from a display element 11, which is reflected by a first mirror 21 onto a curved mirror 22, which reflects it toward the mirror unit 3. The mirror unit 3 is depicted here as the windshield 31 of a motor vehicle. From there, the beam SB2 travels toward an eye 61 of a viewer.

The observer sees a virtual image VB, which is located outside the motor vehicle above the hood or even in front of the motor vehicle. Due to the interaction of the optical unit 2 and the mirror unit 3, the virtual image VB is an enlarged representation of the image displayed by the display element 11. Here, a speed limit, the Current vehicle speed and navigation instructions are displayed. As long as the eye 61 is within the eyebox 62 indicated by a rectangle, all elements of the virtual image are visible to the eye 61. If the eye 61 is outside the eyebox 62, the virtual image VB is only partially visible or not visible at all to the viewer. The larger the eyebox 62, the less restricted the viewer is in choosing their seating position.

The curvature of the curved mirror 22 is adapted to the curvature of the windshield 31 and ensures that the image distortion is stable across the entire eyebox 62. The curved mirror 22 is rotatably mounted by means of a bearing 221. The resulting rotation of the curved mirror 22 enables the eyebox 62 to be moved and thus the position of the eyebox 62 to be adjusted to the position of the eye 61. The first mirror 21 serves to ensure that the path traveled by the beam SB1 between the display element 11 and the curved mirror 22 is long, while at the same time the optical unit 2 remains compact. The optical unit 2 is separated from the environment by a transparent cover 23. The optical elements of the optical unit 2 are thus protected, for example, against dust present in the interior of the vehicle. An anti-glare screen 24 serves to reliably absorb the light reflected across the boundary surface of the cover 23, preventing glare to the viewer. In addition to sunlight SL, light from another interfering light source 64 can also reach the display element 11.

Fig. 2 shows a schematic of the imaging unit 1 of a head-up display. The light source 12 can be seen, the light of which is collimated by a collimator 13. The collimated light beam has a height h in the image plane perpendicular to its propagation direction ABR1. It is reflected by a mirror 14 arranged at an angle of α=45° to the propagation direction ABR1 and, in its propagation direction ABR2, oriented at an angle of 90° to the propagation direction ABR1, illuminates the display element 11, from where it enters the optical unit 2 (not shown here) as a beam SB1. The display element 11 is not perpendicular to the propagation direction ABR2. arranged, but at an angle deviating from 90°, which is shown here as deviating particularly strongly from 90°.

Fig. 3 shows a schematic of the imaging unit of a head-up display according to the invention. The folding mirror 15 according to the invention can be seen, which is arranged at an angle of β<45°. Due to its property that the angle of incidence and the angle of reflection are unequal, the basic propagation direction ABR1 of the light arriving at it from the light source 12 and the light reflected by it in the propagation direction ABR2 toward the display element 11 remains unchanged compared to the previous figure. However, the dimensions of the light source 12 and the collimator 13, as well as the height h' of the collimated light beam, are smaller than in the previous figure. This saves installation space. The height h' in this figure is smaller than the height h in the previous figure. This not only means that the arrangement requires less space, but also that the lighting unit, here the light source 12, only has to generate a flatter light beam, which also makes the light source 12 more compact. Another important space advantage is the greater freedom of positioning the light source while maintaining the desired angle in the display area. The smaller light source also saves space in the implementation, for example, as LEDs.

In the illustration, the distance between display element 11 and folding mirror 15 is shown larger than it actually is for the sake of clarity. The space savings achieved by the invention are primarily evident in the illustration by the height h', which is smaller than the height h in the previous illustration. The illuminated area of display element 11 also appears smaller than in the previous illustration, which is also not the case but is due to the schematic representation.

Fig. 4 shows a schematic view of a folding mirror 15 of a head-up display. The folding mirror 15 has a first boundary surface 151, at which incident light is reflected, and a second boundary surface 152. It can be seen that the folding mirror 15 has at its first interface 151, the reflective surface, many microstructures 16, each of which satisfies the well-known rule that the angle of incidence equals the angle of reflection. Macroscopically, averaged across the microstructures 16, the first interface 151 and the second interface 152 are parallel to one another. The microstructures 16 have first mirror surfaces 161, which in the illustrated embodiment have an angle of 45° to the propagation direction ABR1 and to the propagation direction ABR2. In the propagation direction ABR1, there are gaps 163 between each two mirror surfaces 161. The mirror surfaces 161 are connected to one another in the gaps 163 by inclined second surfaces 162.

Macroscopically, the angle of incidence θ1 of the folding mirror 15 is greater than its angle of reflection θ2, which are indicated relative to the perpendicular on the interfaces 151, 152. In the illustrated embodiment, θ1=90°-β and θ2=β. An incident beam ESB can be seen in the propagation direction ABR1, which, after reflection at the folding mirror 15, leaves the folding mirror 15 as the reflected beam ASB in the propagation direction ASR2.

Pre-collimated light sources, which may consist of several individual light sources arranged side by side, so-called arrays, either shine directly onto diffusers behind the display element, also referred to below as the display, or are deflected beforehand by folding mirrors. In these cases, the macroscopic angle of incidence = angle of reflection. This leads to space conflicts. Particularly strong color and brightness deviations can occur in the corners/edges between array cells. Light-emitting diodes (LEDs) are often used as light sources. The polarization direction of the LED light, which does not match the display polarizer, is deflected by the display and lost. This requires additional components or occurs within the display and heats it up. Tilting the folding mirror by one angle changes the angle of the illuminating light by twice the angle.

Such solutions have the disadvantage of increased space requirements, which limits the image size, the requirement of arrays with relatively high cell counts, Color and brightness inhomogeneities, as well as efficiency losses due to the loss of a polarization component, are desirable and demonstrated by the invention.

The core idea of the invention is a finely stepped folding mirror 15, which macroscopically deviates from the angle of incidence = angle of reflection of a conventional mirror 14, see Fig.4 , and with the further developments described below, polarization recycling is possible. The properties and freedom thus gained open up a whole range of design possibilities with advantages beyond space savings.

This provides particularly space-saving options for folding the beam path of the imaging unit 1 into the installation space. The light distribution is spread by dividing it into strips that are then pulled apart. This allows the original illumination unit to be reduced in size. This helps avoid array boundaries in the image area and improves homogeneity. Design variants according to the invention allow for tolerance-insensitive designs and increased efficiency through polarization recycling.

Fig. 5 shows schematically a part of a first embodiment not claimed here. One can see the folding mirror 15, the upper boundary surface 151 of which has microstructures 16, and the lower boundary surface 152 of which has no special optical or geometric properties essential in connection with the invention. The microstructures 16 have first mirror surfaces 161, which are at an angle of 45° to the propagation direction ABR1 of the incident light. Between the first mirror surfaces 161 there are gaps 163, in which second surfaces 162 are arranged, which are also designed as reflective surfaces. The second surfaces 162 are aligned parallel to the propagation direction ABR1 of the incident light. A retarder 18 and a polarizer 17 are arranged above the folding mirror 15. In the present embodiment, the retarder 18 has the property of a quarter-wave plate, i.e., it converts linearly polarized input light into circularly polarized output light, and vice versa. The polarizer 17 is a reflective polarizer that allows linearly polarized light of a first polarization direction to pass through and reflects light polarized perpendicular to it.

From the left, unpolarized light L1, generated by light source 12 and collimated by collimator 13, falls onto folding mirror 15 in propagation direction ABR1. For the sake of clarity, only one light beam is shown here as an example. This unpolarized light L1 is reflected by mirror surfaces 161. It reaches retarder 18 in propagation direction ABR2 as unpolarized light L2, passes through it, and leaves it as unpolarized light L3. It strikes reflective polarizer 17, which transmits s-polarized light L4s and reflects p-polarized light L4p. For the sake of clarity, this is shown schematically offset to the right in the figure. P-polarized light L4p passes through retarder 18 and leaves it as circularly polarized light L5z. This light strikes the reflective second surface 162 and is reflected by it as circularly polarized light L6z back to the retarder 18. It passes through the retarder and exits it as s-polarized light L7s. This light passes through the reflective polarizer 17 because it now has the polarization direction that it transmits rather than reflects. Thus, further s-polarized light L8s reaches the display element 11.

In the figure, the respective described light Lxn (x=1,2,...; n=p/z/s/_) is drawn parallel to the respective propagation direction ABR1, ABR2, and after reflection at the polarizer 17 or at a reflective second surface 162, is imaged laterally offset. The latter indicates that the light normally does not consist of ideally parallel rays, but of at least slightly divergent rays. These are largely reflected obliquely by the polarizer 17, so that they reach one of the reflective second surfaces 162, where they are reflected again. Additionally or alternatively, the first mirror surfaces 161 can be provided with a curvature, which makes the light L2 reflected by them more divergent than the light L1 incident on them. Further possibilities include undulating or tilting the polarizer. With tilting Advantageously, the inclination of the mirror surfaces 162 is adjusted to minimize the angular deviation. With one or more of these measures, a portion of the light L4s transmitted by the polarizer 17 already fills some of the dark areas in the light traveling toward the display element 11 caused by the gaps 163. On the other hand, light L8s also reaches these dark areas. More of the originally incident light L1 reaches the display element 11 and has a more uniform brightness profile. The display element 11 is located at a distance above the polarizer 17 and is not shown here. The marked area F8 can be advantageously configured according to a variant described further below.

Fig. 6 schematically shows part of a second embodiment not claimed here. Here, the folding mirror 15 is part of a transparent body 19. The transparent body 19 has a wedge-shaped cross-section, which is shown here in cross-section. The tip of the wedge, which is located on the right in the figure, is capped and therefore not shown. The wedge base surface 191 is the light entry surface facing the light source. The microstructures 16 are arranged on one of the large side surfaces 192 of the wedge. The other large side surface 193 of the wedge forms the light exit surface facing the display element 11.

In an embodiment not shown here, the microstructures 16, the polarizer 17, and the retarder 18 are arranged as shown in the previous figure. In the embodiment shown here, the first mirror surfaces 161, as previously described, are arranged at an angle of 45° to the propagation directions ABR1, ABR2. However, the reflective second surfaces 162 are not arranged parallel to the propagation direction ABR1, but are tilted at an acute angle to it. They are inclined such that they do not obstruct the light L1 incident from the left on its path to one of the first mirror surfaces 161, but are inclined away from one first mirror surface 161 to the next, viewed in its propagation direction. The other large side surface 193 of the wedge-shaped transparent body 19 has the same inclination as the reflective second surfaces 162. This can be seen from the acute angle between the normal 193N of the side surface 193 and the Propagation direction ABR2. The polarizer, designed here as a reflective circular polarizer 172, which also combines the function of the retarder 18, is arranged on the side surface 193 and thus has the same inclination. The first large side surface 192 is provided with a mirror coating.

From the left, unpolarized light L1, generated by light source 12 and collimated by collimator 13, falls onto folding mirror 15 in propagation direction ABR1. For the sake of clarity, only a few light rays are shown here as examples. This unpolarized light L1 is reflected by mirror surfaces 161. It reaches reflective circular polarizer 172 in propagation direction ABR2 as unpolarized light L2. This polarizer transmits s-polarized light L4s and reflects circularly polarized light L5z. Due to the slight tilt of the perpendicular on side surface 193 to propagation direction ABR2, this circularly polarized light L5z propagates at an angle deviating from 0° to propagation direction ABR2. This light strikes the reflective second surfaces 162 and is reflected back to retarder 18 as circularly polarized light L6z. Due to the inclined arrangement of the reflective second surfaces 162, it now propagates again parallel to the propagation direction ABR2. It strikes the reflective circular polarizer 172 and is transmitted by it. Thus, further s-polarized light L8s reaches the display element 11.

The figure is thus also an example of the variant of the invention in which the reflective polarizer 172 is inclined at an angle different from 90° to the propagation direction ABR2 of the light L2 coming from the folding mirror 15 and incident on it, and the reflective second surfaces 162 are arranged parallel to the reflective polarizer 172.

Fig. 7 shows schematically a part of a third embodiment not claimed here. Here, the folding mirror 15 has a first interface 151 and a second interface 152, both of which are arranged parallel to one another and have microstructures 16, 16' arranged offset from one another. The offset is selected in the illustrated embodiment such that in the propagation direction ABR1 of the light L1 coming from the light source 12, first mirror surfaces 161 of the first interface 151 and first mirror surfaces 161' of the second interface 152 follow one another. In the propagation direction ABR2 perpendicular thereto, first mirror surfaces 161 of the first interface 151 and second surfaces 162' of the second interface 152 follow one another, as do second surfaces 162 of the first interface 151 and first mirror surfaces 161' of the second interface 152. The mirror surfaces 161 of the first interface 151 are designed as a reflective polarizer 17. The second surfaces 162 of the first interface 151 are designed as a retarder 18 that rotates the polarization direction by 90°. The first mirror surfaces 161' of the second interface 152 are designed as mirrors that do not influence the polarization. The first mirror surfaces 161, 161' are arranged at an angle of 45° to both the propagation direction ABR1 of the light L1 coming from the light source 12 and the propagation direction ABR2 of the light traveling to the display element 11. The second surfaces 162, 162' are arranged parallel to the propagation direction ABR1 of the light L1 coming from the light source 12.

From the left, unpolarized light L1, generated by light source 12 and collimated by collimator 13, falls onto folding mirror 15 in the propagation direction ABR1. For clarity, only one light beam is shown here as an example. This unpolarized light L1 is reflected by mirror surfaces 161 as s-polarized light L2s and transmitted as p-polarized light L2p. The s-polarized light L2s travels in the propagation direction ABR2 toward the display element 11. The p-polarized light L2p is reflected by the first mirror surfaces 161' of the second interface 152 and travels as p-polarized light L3p from the inside to the second surfaces 162 of the first interface 151. Since these are designed as retarders 18 that rotate the polarization by 90°, they transmit the light incident on them, which leaves them as s-polarized light L4s in the propagation direction ABR2 in the region of the gaps 163. Thus, further s-polarized light L4s travels toward the display element 11.

Fig.8 shows the Fig.5 marked area F8 in an advantageous variant. Instead of a single surface, there are two reflective second surfaces 1621,1622 which are arranged at right angles to each other. They therefore act as retroreflectors. Incident light L5z is always reflected in such a way that the outgoing light L6z is aligned parallel to it. While in the Fig.5 In the variant shown, the incident and outgoing light are only parallel to each other when the incident light hits the reflecting surface 162 at exactly a right angle, this is also the case with a retroreflector when there is an angular deviation.

Fig.9 shows an imaging unit 1 similar to Fig.3 described. Here, however, the display element 11 is arranged at the same angle β as the folding mirror 15. The folding mirror 15 and the display element 11 are thus aligned parallel to one another. Light rays coming from the folding mirror 15 that strike the display element 11 therefore have a path of equal length. Uniform illumination of the display element 11 is thus achieved, since deviations caused by differences in path distance do not occur in this constellation. This is particularly advantageous when using a flat display element 11 that consists, for example, of a single flat light element, or of an LED array in which several LEDs are distributed over a surface. This advantage also arises when, in a head-up display, a folding mirror with the property that the angle of incidence and angle of reflection are macroscopically unequal is aligned parallel to a display element of this head-up display.

Fig.10 shows schematically a part of an embodiment not claimed here with a curved folding mirror 15. The folding mirror 15 is not curved from a macroscopic perspective, but the microstructures 16 are arranged such that the folding mirror 15 acts like a cylindrical mirror. The folding mirror 15 has two mutually parallel interfaces 151, 152. The upper interface 151 is the interface that the incident light L1 reaches first. The upper interface 151 has first mirror surfaces 161-i with i=1, 2, 3,..., of which the mirror surfaces 161-1, 161-2 and 161-3 are shown in the exemplary embodiment shown here. The upper interface 151 has second surfaces 162-i with i=1, 2, 3,..., of which the second Surfaces 162-1, 162-2, and 162-3 are shown. These are located in gaps 163 between the mirror surfaces 161-i. The lower interface 152 is the one impinged upon by light L2p transmitted through the upper interface 151. The lower interface 152 has mirror surfaces 161'-1, 161'-2, and 161'-3.

The first mirror surface 161-1 of the first mirror surfaces 161-i shown has a first angle δ1 to the propagation direction ABR1, the first playing surface 161-2 has a first angle δ2 to the propagation direction ABR1, and the first playing surface 161-3 has a first angle δ3 to the propagation direction ABR1. The first angles δ1, δ2, and δ3 differ slightly from one another. Thus, each of the first mirror surfaces 161-i has a different first angle δi. The first mirror surface 161'-1 of the lower boundary surface 152 has a second angle φ1 to the propagation direction ABR1. The first mirror surface 161'-2 has a second angle φ2 to the propagation direction ABR1. The first mirror surface 161'-3 has a second angle φ3 to the propagation direction ABR1. Thus, each of the first mirror surfaces 161'-i of the lower boundary surface 152 has a different second angle φi than the other first mirror surfaces 161'-i. The first angles δi and the second angles φi decrease from left to right in the illustrated embodiment. The following applies:
δ1<φ1<δ2<φ2<δ3<φ3. Thus, the value of every second angle φi lies between the values of the first angles δi of two adjacent first mirror surfaces 161-i. The light rays Lxs exiting the folding mirror 15 upwards are thus spread out evenly. The second surfaces 162-i, 162-i' are arranged parallel to the propagation direction ABR1 of the light L1 coming from the light source 12.

The mirror surfaces 161-i of the upper boundary surface 151 are designed as a reflective polarizer 17. The second surfaces 162-i of the first boundary surface 151 are designed as a retarder 18 that rotates the polarization direction by 90°. The first mirror surfaces 161'-i of the lower boundary layer 152 are designed as mirrors that do not influence the polarization.

From the left, unpolarized light L1, generated by light source 12 and collimated by collimator 13, falls onto folding mirror 15 in the propagation direction ABR1. For clarity, only a few light rays are shown here as examples. This unpolarized light L1, L5 is reflected by mirror surfaces 161-i as s-polarized light L2s, L6s and transmitted as p-polarized light L2p, L6p. The s-polarized light L2s, L6s travels upwards towards the display element 11. The p-polarized light L2p, L6p is reflected by the first mirror surfaces 161'-1, 161'-2 of the second interface 152 and travels as p-polarized light L3p, L7p from the inside to the second surfaces 162-i of the first interface 151. Since these are designed as retarders 18 that rotate the polarization by 90°, they transmit the light incident on them, which leaves them as s-polarized light L4s, L8s upwards in the region of the gaps 163 in the figure. Thus, further s-polarized light L4s, L8s travels towards the display element 11. It can be seen that the light Lxs traveling towards the display element 11 consists of fanned-out light rays.

Fig.11 schematically shows part of an embodiment according to the invention with a curved folding mirror 15. Here, too, the folding mirror 15 is not macroscopically curved, but the microstructures 16 are arranged such that the folding mirror 15 acts like a cylindrical mirror. The upper boundary surface 151 of the folding mirror 15 has microstructures 16, and its lower boundary surface 152 has no special optical or geometric properties essential in connection with the invention. The microstructures 16 have first mirror surfaces 161-i, of which the mirror surfaces 161-1 to 161-5 are shown. Each of the first mirror surfaces 161-i has a first angle δi to the propagation direction ABR1 of the incident light, which differs from the other first angles δi. Between the first mirror surfaces 161-i are gaps 163, in which second surfaces 162 are arranged, which are also designed as reflective surfaces. Instead of a single surface, two reflecting second surfaces 1621, 1622 are provided as second surfaces 162, which are arranged at right angles to each other. They therefore act as a retroreflector. A retarder 18 and a polarizer 17 are arranged above the folding mirror 15. In the present case, the retarder 18 has

In this embodiment, the polarizer 17 has the properties of a quarter-wave plate, meaning it converts linearly polarized input light into circularly polarized output light, and vice versa. Polarizer 17 is a reflective polarizer that allows linearly polarized light of a first polarization direction to pass through and reflects light polarized perpendicular to it. Instead of a single surface, the polarizer has a plurality of paired polarizer surfaces 1701, 1702, which are arranged at right angles to each other. They therefore act as a retroreflector.

From the left, unpolarized light L1, generated by light source 12 and collimated by collimator 13, falls onto folding mirror 15 in propagation direction ABR1. For the sake of clarity, only a few light rays are shown here as examples. This unpolarized light L1 is reflected by mirror surfaces 161-3. It travels upwards as unpolarized light L2 to retarder 18, passes through it, and exits it as unpolarized light L3. It strikes reflective polarizer 17, designed as a retroreflector, which transmits s-polarized light L4s and reflects p-polarized light L4p. The p-polarized light L4p passes through retarder 18 and exits it as circularly polarized light L5z. This light strikes one of the reflective second surfaces 162, also designed as a retroreflector, and is reflected back to the retarder 18 as circularly polarized light L6z. It passes through the retarder and exits as s-polarized light L7s. This light passes through the reflective polarizer 17 because it now has the polarization direction that it transmits rather than reflects. Thus, further s-polarized light L8s reaches the display element 11.

According to a further variant not described in detail here, a transparent body 19 as shown in Fig.6 described, which is provided with corresponding microstructures. The function of the reflective polarizer 17 and the retarder 18 is realized there in a circular polarizer 172, which is designed as a retroreflector by means of first and second polarizer surfaces 1701 and 1702.

In other words, the invention relates to a head-up display with an efficient, cost- and space-saving imaging unit 1, which has a folding mirror 15 in which the angle of incidence and the angle of reflection are macroscopically unequal. Such a unit is also referred to as a "blazed-mirror PGU" (PGU: Picture Generating Unit). The invention relates to the field of head-up displays (HUDs) and other display systems that use directed light of a specific polarization, for example, based on liquid crystal display elements. Most current LED-based TFT HUDs (TFT: Thin Film Transistor, a variant of liquid crystal displays) have an almost complete loss of light output for one polarization component of the illumination light. This light output loss is typically implemented in an external polarization filter to reduce the heating of the display element 11. The first systems on the market and in the literature that reuse portions of the "incorrectly" polarized light are already available. These more efficient systems require additional components and thus additional installation space and costs. Conventional systems require more energy, which is becoming increasingly important (e.g., electric mobility range). The thermal load becomes increasingly critical for the systems/components, especially with increasing image sizes. Desired, individually or in combination, are increased efficiency, reduced installation space or enlargement of the virtual image while maintaining the same installation space, cost reduction, and homogenization of the illumination. At least one of these is achieved by the invention.

When using a folding mirror with unequal angles of incidence and reflection (blazed mirror), stripes are created from the perspective of the display element 11, from which no light emanates. These areas, the gaps 163, are used according to the invention to reuse light from the unwanted polarization components by changing their polarization state and redirecting it to the display element 11, thus making it usable for its backlighting. The HUD is therefore cheaper, has lower energy, space, and/or cooling requirements, and offers more homogeneous illumination. The invention is also more generally applicable to other Efficiency-sensitive backlit display elements with a narrower viewing angle. The invention is also applicable to projection systems, for example, based on DMD technology (DMD: Digital Micromirror Device - the device's light deflection is based on one or more digitally controlled micromirrors) or LCoS technology (LCoS: Liquid Crystal on Silicon - light modulation is based on liquid crystals mounted on a silicon substrate). This is then a display unit with an imaging unit 1 for generating an image, wherein the imaging unit has a folding mirror, the folding mirror is arranged between a light source and a display element illuminated by the light source at an angle of incidence to the propagation direction of the light incident on it from the light source, the folding mirror has microstructures, wherein the microstructures have first mirror surfaces which are arranged at a first angle different from the angle of incidence of the folding mirror and are spaced from one another to form gaps, wherein second surfaces are arranged at a second angle in the gaps, a polarizer guides light of a first polarization to the display element and guides light of a second polarization into the gaps, a retarder converts the polarization of the light guided into the gaps into the first polarization, and the light guided into the gaps is guided towards the display element after passing through the gaps. According to a variant of the invention, the light of undesired polarization is reflected by a reflective circular polarizer 172 under the display to the folding mirror 15, where it is reflected in the previous dead zones, the gaps 163, then converted into the useful direction by the circular polarizer 172 and thus reused.

According to one variant, the angle at which the folding mirror, also known as a blazed mirror, is arranged is optimized for a tilt of the display element. One advantageous embodiment consists in aligning the blazed mirror as parallel as possible to the display element. This results in the most even blurring of its structures. In image-generating units with LED arrays currently in use, there are sometimes significant relative differences in the distance between the LEDs and a scattering element arranged below the display element because the display element is tilted. This leads to Varying visibility of the LED array. This undesirable effect is greatly reduced by aligning the folding mirror and display element in parallel. Equal distance between the display element and the folding mirror means smaller relative path differences.

An astigmatism function, i.e., a fanning out of the light reaching the display element 11, is achieved according to variants of the invention as follows. The angles δi of the surfaces 161-i, which reflect the light incident from the left, gradually change their angular value. Thus, there is no longer a folding mirror 15 in the narrower sense, but the folding mirror 15 thus formed is curved in one direction. This is a so-called blazed cylindrical mirror. However, this may be faceted, i.e., equipped with flat mirror surfaces 161-i, as in, for example, Fig.11 There, after reflection at the folding mirror 15, the light is no longer incident perpendicularly on the reflecting polarizer 17. The reflected light beam would therefore diverge. Therefore, in the exemplary embodiment, the Fig.11 It is planned to design both reflective structures as retroreflectors so that the recycled light also has the desired radiation direction to the respective location.

Claims (3)

1. Head-up display for a means of transportation, comprising:

   - a picture generating unit (1) for generating an image;

   - an optical unit (2) for projecting the image by means of a mirror unit (3),
   wherein

   - the picture generating unit (1) has a folding mirror (15),

   - the folding mirror (15) is arranged between a light source (12) and a display element (11), through which the light source radiates light, at a work angle (β) to the propagation direction (ABR1) of the light (L1) that is incident on the folding mirror from the light source (11),

   - the folding mirror (15) has microstructures (16),

   -- wherein the microstructures (16) have first mirror surfaces (161,161-i,161',161'-i) that are arranged at a first angle, which deviates from the work angle (β) of the folding mirror (15), and are spaced apart from one another to form gaps (163),

   -- wherein second surfaces (162,162-i,162',162'-i) are arranged in the gaps (163) at a second angle,

   - the picture generating unit (1) has a polarizer (17, 172) and a retarder (18),
   characterized in that
   the polarizer (17, 172) is configured to guide light having a first polarization to the display element (11) and to guide light having a second polarization into the gaps (163),

   - the retarder (18) is configured to convert the polarization of the light (L2p, L4p) guided into the gaps (163) to the first polarization, and

   - the second surfaces (162) are configured to reflect the light guided into the gaps (163) in the direction of the display element (11), wherein

   - the polarizer (17) is designed as a reflective polarizer (17) and is arranged between the folding mirror (15) and the display element (11), and

   - the retarder (18) is designed as a retarder (18) converting linear to circular polarization and is arranged between the folding mirror (15) and the polarizer (17), wherein the reflecting second surfaces (1621,1622) each form in pairs a retroreflector, wherein

   - the first mirror surfaces (161-i) have first angles (δi) that differ from one another, and

   - the reflective polarizer (17) has first polarizer surfaces (1701) and second polarizer surfaces (1702), which each form in pairs a retroreflector.
2. Head-up display according to Claim 1, wherein the folding mirror (15) is part of a transparent body (19) with a wedge-shaped cross section, in which the wedge base area (191) is the light entry surface facing the light source (12), the microstructures (16) are arranged on one of the large side faces (192), and the other large side face (193) is the light exit surface facing the display element (11).
3. Head-up display according to either of the preceding claims, wherein the folding mirror (15) and the display element (11) are aligned parallel to each other.